Such a bottom structure for a vehicle body typically comprises left and right side sills, left and right rear side frames extending from a rear part of the left and right side sills toward a rear part of the vehicle body, a first cross member that extends between the left and right side sills and that is connected to front end parts of the left and right rear side frames, and a second cross member that is rearward of the first cross member and extends across the left and right rear side frames so as to be positioned between left and right rear wheels.
In the bottom structure for a vehicle body, the left and right rear side frames, first cross member, and second cross member form a rectangular frame. Accordingly, a force from the rear wheels acts upon the second cross member of the rectangular frame in a width direction of the vehicle when the vehicle turns. For this reason, the bottom structure for a vehicle body must be kept rigid in order to prevent the rectangular frame from being deformed by the force from the rear wheels.
Therefore, a structure for a vehicle body is proposed in JP-A-2005-119492 that comprises a plurality of cross members in the vicinity of the second cross member in order to keep the bottom structure for a vehicle body rigid.
However, the bottom structure for a vehicle body of JP-A-2005-119492 comprises a plurality of cross members in the vicinity of the second cross member. Therefore, the bottom structure for a vehicle body has a complex configuration, which prevents the bottom structure from being made lighter.